Nitrogen plays an important role in increasing the pasture and crop yields helping to fill the feed gaps. When used
strategically, nitrogen is effective in synthesis of proteins in both plants and animals. Nitrogen is also an essential
component of green pigment (chlorophyll) in plant leaves, which are vital for photosynthesis.
To get the most effective benefit of using nitrogen in farming systems, it is important to understand the nitrogen
cycle.
With the coldest months of the year upon us, farmers should be particularly cautious when applying nitrogen fertilisers
to pasture or crops during winter.
Winter applications of nitrogen fertilisers are generally least effective for promoting grass growth. That’s because
slow growth of pasture in winter and greater drainage can result in nitrate leaching before plants can take it up. The
nitrogen can make its way to waterways where it can stimulate nuisance algal growth.
Lactating cows will excrete, in urine, about 70 per cent of the nitrogen they consume. Again, the risk of this nitrogen
leaching from urine patches is much higher in winter.
This nitrogen leaching, along with phosphorus run off, not only contaminates waterbodies but is a loss of economically
valuable nutrients.
Nutrient budgeting using computer models such as Overseer, combined with feed budgeting, enables farmers to understand
if they are using too much or too little fertiliser. By doing this, farmers can optimise the use of nutrients and reduce
the impact on the environment by developing a pragmatic nutrient management plan.
Understanding the term “response rate” helps farmers when it comes to implementing these plans.
The response rate is the amount of pasture grown in terms of kilograms of dry matter per hectare per kilogram of
nitrogen (N) applied. For example, when 20kg N/ha is applied and an additional 200kg DM/ha of pasture is grown the
response rate is 10kg DM/kg N applied. The response is dependent on several factors such as soil temperature, plant
growth, soil moisture, the deficiency of available nitrogen in the soil and the rate of nitrogen applied per
application.
The best response to N fertiliser occurs on fast growing pasture, when other factors such as moisture and soil
temperature are not limiting growth. Response rate variation also depends on the season and on nitrogen application
rate. In winter, at the same application rate, responses are lower and slower than other times of the year.
It is better to apply nitrogenous fertiliser when the pasture cover is around 1500kg DM/ha. This ensures there is
sufficient leaf area for photosynthesis leading to good pasture growth.
Also, nitrogen fertiliser reduces nitrogen fixation by clover by about one kg N/ha/year for every 3kg of nitrogen
fertiliser applied. In addition, clover content will be further reduced if nitrogen-boosted pastures shade the clover.
This effect is seen during spring.
Remember that the profitability of applying nitrogen is dependent on the utilisation of the extra feed. Therefore,
nitrogen needs to be strategically applied to fill genuine feed deficits.
Nitrogen conversion efficiency for any farm is another key point to be remembered. This is measured by calculating total
nitrogen in product divided by the total nitrogen inputs into a farm and is expressed as a percentage. A dairy farm, for
example, is probably doing fine with about 40 per cent.
A number of farmers, as well as industry organisations, are already doing a great job of trying to increase productivity
and reduce environmental impacts through more careful use of nutrients. The more we can share information on the best
way to do these things the better.
I suggest getting clear advice about the risks involved with winter nitrogen applications on individual properties.
• Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council. Contact him on 0800 800 401 or
at bala.tikkisetty@waikatoregion.govt.nz